Calvin cycle reduction phase: During the reduction phase of the Calvin–Benson cycle, 3-phosphoglyceric acid (3-PGA) is reduced to which compound, and which reductant supplies the reducing power?

Introduction / Context:
The Calvin–Benson cycle fixes CO2 into organic molecules. After carboxylation by RuBisCO, the reduction phase converts 3-phosphoglyceric acid (3-PGA) into triose phosphates that serve as precursors for carbohydrates and other biomolecules. Recognizing the product and reductant is essential for linking photosynthetic light reactions to carbon assimilation.

Given Data / Assumptions:

• Starting intermediate: 3-PGA.
• Photosynthetic organisms use NADPH generated by light reactions.
• Product commonly termed glyceraldehyde-3-phosphate (G3P) or phosphoglyceraldehyde (PGAL).

Concept / Approach:

3-PGA is first phosphorylated by ATP to 1,3-bisphosphoglycerate and then reduced by NADPH + H+ to glyceraldehyde-3-phosphate (phosphoglyceraldehyde). NADPH provides the high-energy electrons derived from photosystem I, directly coupling light energy to CO2 reduction.

Step-by-Step Solution:

Verification / Alternative check:

Textbook stoichiometry for one CO2 fixed requires ATP and NADPH; NADPH specifically supplies the reducing equivalents in chloroplasts and many photoautotrophic bacteria.

Why Other Options Are Wrong:

NADH + H+: typically used in catabolic pathways; Calvin cycle reduction uses NADPH.

Ribulose-1,5-bisphosphate is the CO2 acceptor regenerated later, not the reduction product of 3-PGA.

Pyruvic acid is not a Calvin cycle intermediate.

Common Pitfalls:

Confusing G3P (PGAL) with 3-PGA; mixing up NADH and NADPH roles; forgetting the ATP-dependent phosphorylation preceding the reduction step.

Final Answer:

phosphoglyceraldehyde; NADPH+H+